In many applications, the sensing, manipulation, and display of optical information are based on scattering of optical fields with specific frequencies on various scattering structures. These actions are significantly more efficient when the scatterer is brought into a resonant regime.
Metals, or more generally conductors, exhibit special optical properties due to the existence of coherent charge density oscillations. On surfaces these take the form of propagating waves known as surface plasmons. Similarly, conductive structures can exhibit collective electron oscillations at frequencies determined by the shape, size, composition, crystallinity, and environment of the structure. For example, in metal nanoparticles these collective oscillations lead to pronounced resonances at optical frequencies. These resonances become apparent in extinction measurements due to resonantly enhanced scattering and absorption. A prior art example is shown in the graph of FIG. 1, which shows experimental data on the optical transmission of 41 nm diameter silver nanoparticles in an aqueous solution. The extinction peak at approximately 410 nm is caused by the plasmon resonance in the silver nanoparticles.
Associated with these resonances are localized electromagnetic fields with significantly enhanced field strength relative to that of the excitation source. An example is shown in FIG. 2, which shows a calculated snapshot 200 of the energy density around a metal nanoparticle under plane-wave illumination at the surface plasmon resonance frequency. Field enhancements can be as large as several orders of magnitude. The ability to actively control these resonances could play an important role in applications that benefit from enhanced electric fields, enhanced localization of light, enhanced optical absorption, or enhanced optical scattering.
One application in which this field enhancement plays an important role is Surface Enhance Raman Spectroscopy (SERS). Raman scattering involves the excitation or annihilation of phonons upon laser illumination of certain materials, resulting in the appearance of photons with a slightly modified energy. By measuring the energy distribution of the scattered laser light, information can be obtained about the structure of the object, allowing for identification of the scattering element.
Although Raman scattering has been shown to yield identifiable molecular vibration spectra, the signal strength is typically low, making the detection of low concentrations of molecules difficult. It has been found however that the strength of the Raman signal from a given molecule can be enhanced by factors in excess of 1010 near the surface of a resonantly excited metal nanoparticle, as discussed by Michaels et al. in Surface Enhanced Raman Spectroscopy of Individual Rhodamine 6G Molecules on Large Ag Nanocrystals, J. Am. Chem. Soc. 121, 9932 (1999). The magnitude of the signal enhancement makes SERS a viable technique for performing high sensitivity chemical detection.
Surface Enhance Raman Scattering can occur at frequencies where metal nanostructures exhibit plasmon resonances. In experiments, an excitation wavelength as short as possible is chosen because the strength of the Raman signal depends to the fourth power on the frequency of the excitation source. However, at short excitation wavelengths, many molecules show bright fluorescence that can make the weak Raman signal difficult to detect. Consequently, for each molecule, it is necessary to determine the shortest possible excitation wavelength and design the corresponding metal structure to obtain SERS at that wavelength.
A second example where tunability is desired is the case of Surface Enhanced Resonant Raman Scattering (SERRS). It has been observed that Raman signals can be further enhanced in specific situations where the excitation wavelength overlaps with an absorption band of the species under investigation. In these cases, the resonance of the metal nanostructure must be tuned to match the energy of the desired molecular transition, requiring a broad range of resonance frequencies to be covered.
SERRS requires a tailored conductive nanostructure, e.g. a metal nanoparticle, for each species that needs to be detected. This approach is feasible when a single species needs to be detected, but rapidly becomes impractical when multiple species are involved. In the latter case, it would be beneficial to be able to tune the resonance frequency of the metal nanostructure to coincide with several different excitation wavelengths, enabling the detection of several species within a small detection volume. This would allow the design of compact biochemical sensors for multi-species detection.
Thus, there exists a need to be able to actively tune the resonant frequency of a metal nanoparticle to a broad range of frequencies that has not been achieved in the prior art.